1. Field of the Invention
The invention relates to a heatable glazing or calendering roll in which a fluid heat carrier flows through an annular gap between a displacement body and a cylindrical hollow body.
2. Description of the Prior Art
Such a roll known for example from DE-OS No. 3 014 891, and also from the prior U.S. patent application Ser. No. 863,261 is used, in particular, for making and processing paper. In this connection, the surface temperature of the roll has to be influenced frequently, that is to say its surface has to be heated or cooled. In the majority of cases this is done by means of a fluid heat carrier flowing through the roll, that is steam, water or oil; this heat carrier is heated or cooled outside the roll and is then guided through the roll.
In order to obtain good heat transfer the fluid heat carrier has to approached to the surface of the roll as closely as possible. For this purpose the central bore of the cylindrical hollow body is broadened so that a cylindrical displacement body can be inserted into the central bore and the fluid heat carrier flows through a narrow annular gap between the displacement body and central bore.
Examinations of such heatable glazing or calendering rolls have shown, however, that despite the use of fluid heat carriers, for instance, of water or oil, the surface of the roll does not have a uniform temperature, due to which, for one thing, the web material to be processed is influenced in a negative way and, for the other, the shape of the roll varies adversely. As early as in the 60's the influence of shape changes of the glazing roll, due to axial and radial temperature differences, on the roll profile and thus also on the paper profile was examined (cf. the lecture "Improving the Paper Profile and the Gloss by Heated Glazing and Calendering Rolls" given by Peter Rothenbacher, Erich Vomhoff and Michael Zaoralek at the main congress of the OZEPA in Klagenfurth on Oct. 18, 1984). If in accordance with a usual rule of thumb for the heat expansion of iron and/or steel at a temperature difference of 1.degree. C. and a reference length of 1000 mm a diameter change of about 10 .mu.m is assumed, a temperature change of 4.degree. C. in a roll having a rated diameter of 710 mm will manifest itself in an increase in the diameter of 15 .mu.m. Such minor temperature fluctuations occurring at points with different flow rates due to the involved different heat transfer coefficients and the roll's shape changes resulting therefrom cannot be kept under control even by carefully setting the temperature of the fluid heat carrier.